Reduced Fire Frequency Changes Species Composition of a Ponderosa Pine Stand
Alan Dickman. 1978. Reduced Fire Frequency Changes Species Composition of a Ponderosa Pine Stand. Journal of Forestry, January 1978.
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Selected excerpts:
Abstract
In the Umpqua National Forest, Oregon, a 35-acre ponderosa pine (Pinus ponderosa Laws.) stand situated in the midst of a Douglas-fir (Pseudotsuga menziesii [Mirb.]Franco) forests is being invaded by Douglas-fir seedlings as a result of reduced fire frequency within the last 50 years. In earlier times frequent ground fires kept Douglas-fir at a minimum.
Pine Bench, an area on the Umpqua National Forest, Oregon, is undergoing a drastic change in species composition. The understory, which according to an early settler, Jessie Wright (personal communication, 1975), was open and grassy until a half-century ago, now contains thickets of Douglas-fir that are shading out seedlings of the overstory ponderosa pines. A study was made to determine the cause and extent of this shift. …
Results
The size-class distribution (fig .1) shows that among the large trees there are far more ponderosa pines than Douglas-firs, while among the small trees there are far more Douglas-firs than ponderosa pines. The age class distribution (fig. 2) shows that the change occurred rather suddenly. …
The linear regressions show that Douglas-fir grows faster than ponderosa pine on Pine Bench; Douglas-fir appearing in the same size-class is actually younger. Therefore, the difference in the large number of old ponderosa pine and the small number of old Douglas-fir is actually even greater than the size-class distribution indicates.
The shift in, species composition began, then, when the middle-aged trees were seedlings. The number of Douglas-fir germinating and surviving was relatively small and stable until 1925, but thereafter increased steadily up to the present.
Discussion
The change seems too quick and drastic to be a result of natural succession. Grazing does not seem responsible, either. According to Jessie Wright (personal communication) cattle were driven through Pine Bench from 1917 to 1952 on their way between summer and winter grazing areas. The cattle were never on the bench long, however, and their impact was slight. Furthermore, Mrs. Wright told me that they grazed fir in preference to pine.
Reduced fire frequency seems the most likely cause of the invasion. …
Two factors may have combined to reduce the frequency of fires on Pine Bench in this century. First is the absence of Indian or settler-caused fires, although as early as 1840 the number of Indians in the North Umpqua Valley was very small (Bakken 1970). An equally likely cause is the suppression of fires by the U.S. Forest Service.
By 1920, a Forest Service fire lookout was established on Illahee Rock, only four miles from Pine Bench, although it was not until the introduction of aerial fire-fighting techniques that control became highly effective. Douglas-firs living through the 1920s and 1930s would have almost been assured of survival once the more effective fire suppression of later decades began.
Prescribed burning has been proven valuable and workable in maintaining ponderosa pine stands (Weaver 1964, 1965) and should be considered for Pine Bench.
